Wood preservatives have been used for more than a century. They are broadly classified as either waterborne or oil-type, based on the chemical composition of the preservative and the carrier used during the treating process. Exemplary commercial waterborne preservatives include Chromated Copper Arsenate (CCA), Ammoniacal Copper Zinc Arsenate (ACZA), Alkaline Copper Quaternary compounds (ACQ), Copper Azole (CA), Copper HDO (CuHDO), and borates.
Among the waterborne preservatives, ACZA is known to be particularly effective to penetrate Douglas-fir and various other difficult-to-treat soft wood species. It is used frequently in the Western United States, and protects wood against attack by decay fungi, insects, and most types of marine borers. ACZA typically is leach-resistant and can be used in a range of commercial and industrial applications. It is known that ACZA treated wood has protection both above ground and in ground contact, as well as in fresh and salt water environment.
ACZA contains copper (II), zinc, and arsenic ions in an ammoniacal solution and is a refinement of the original formulation ammoniacal copper arsenate (ACA), which is no longer available on the market. The method of preparing ACZA type of preservatives is known in the art. For example, U.S. Pat. No. 3,945,835 discloses the preparation of ammoniacal copper zinc arsenic solutions that contain a water-repellant additive by mixing zinc/copper carbonates or zinc/copper arsenate/arsenite with an organic acid and aqueous ammonia solution.
U.S. Pat. No. 4,038,086 discloses a method for making ammoniacal copper zinc arsenate solutions by mixing CAA and ZAA wherein CAA refers to a copper-ammonia-additive system containing As (V), and ZAA refers to a zinc-ammonia-additive system containing As (III) or As (V). This method is cumbersome as it involves the separate preparation of the copper-ammonia-additive-system and the zinc-ammonia-additive system, which requires duplicate reaction apparatus and processing steps. In addition, accordingly to the patentees, the concentrates prepared from this method contain zinc or a combination of zinc and copper ranging from about 4 up to about 15% by weight based on the volume of the preservative solution. This concentration is less than what otherwise might be desirable.
At present, the commercial ACZA wood preservative concentrate on the market is prepared at a wood treating facility by mixing arsenic acid, cuprous oxide, zinc oxide, and ammonium bicarbonate in aqueous ammonia. The concentrate is subsequently diluted to prepare a wood treating solution. Although it is a one step process to prepare ACZA concentrate using this technology, the process requires heat and addition of air to the reaction mixture in order to oxidize the cuprous oxide to the desired cupric compound. Addition of air to the reaction mixture is typically achieved by sparging air through the reaction mixture. Such sparging can result in the loss of ammonia as vapor/gas to the environment. The ammonia loss not only increases the cost of preparing ACZA, but also creates an environmental hazard as it is known that exposure to gaseous ammonia can pose a health hazard to humans and to aquatic animals.
In addition, the commercial process to prepare ACZA concentrate takes about 4 to 5 days for the reaction to complete. Further, the ACZA concentrate produced by the commercial method typically contains from 10 to 12% actives expressed as total weight percentage of CuO, ZnO and As2O5 based on the weight of the concentrate. Unfortunately, it is not as economical as might be desired to ship ACZA with such a low level of actives. For this reason, currently, the ACZA concentrate is produced on site at wood treating facilities rather than at chemical manufacturing facilities.
Accordingly, there exists a need in the wood preservatives manufacturing community for a super concentrated ACZA composition that can be produced in a one step reaction that does not entail sparging with oxygen or air, which can be completed in relatively short time, as compared to the conventional commercial process for making more diluted concentrates. This invention provides an answer to that need.